Temperature dependence study of five-coordinate complex formation of zinc(II) octaethyl and tetraphenylporphyrin

doi:10.1016/S0020-1693(00)00280-2

Inorganica Chimica Acta

Volume 310, Issue 2, 15 December 2000, Pages 175-182

https://doi.org/10.1016/S0020-1693(00)00280-2 Get rights and content

Abstract

Five coordinate complex formation of zinc(II)–octaethylporphyrin (Zn(OEP)) and zinc(II)–tetraphenylporphyrin (Zn(TPP)) have been studied in the presence of seven systematically selected electron donor molecules. Stability constants in toluene were determined at various temperature ranged between 20 and 60°C by absorption and steady-state fluorescent measurements from which thermodynamic parameters were determined. Depending on the porphyrin and the axial ligand the entropy is changing between −127 and −61 J mol⁻¹ K⁻¹ while the enthalpy is ranging from −49 to −24 kJ mol⁻¹.

Introduction

A great number of experimental results have been reported over the past three decades concerning the complex formation of metalloporphyrins with S, O, P and N bases [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. This considerable attention has been drawn to porphyrins, as the molecules specially emerged in evolution for harvesting and converting natural light in living species, because of their excellent photophysical and photochemical characteristics, which can be utilized, besides many other applications in designing artificial solar energy converting systems [17], [18], [19], [20], [21], [22], [23] and in the treatment of tumors by photodynamic therapy (PDT) [24], [25], [26]. The interaction of metalloporphyrins with donor molecules either in their ground or excited state can strongly influence the absorption properties and the efficiency of energy or electron transfer processes of porphyrin derivatives. Numerous studies have attempted to find linear correlation between the chemical characteristics of the electron donor molecule (DN, pK_a, Drago Parameters) and some measured properties (ΔH, ΔG, Soret shift) of the equilibration [10], [27], [28], [29], [30]. Although each author found an overall trend, all of them drew the conclusion that the linearity of any of the correlations was not strict enough to obtain quantitative predictions.

Other authors investigated the characteristics of bonding from the nitrogen of the Lewis base to the metal atom of the porphyrin, also resulting in somewhat ambiguous findings. Concerning the substituent effects of magnesium and nickel porphyrins and also that of the pyridine ring in the fourth position, Storm et al. [3] suggested that d–π bonding between transition metals in porphyrins and pyridine ligands is not very important. However, Cole, Curthoys and Magnusson [31] have investigated the importance of π bonding in the case of porphyriniron(II)–pyridine complexes and set the following order among these donors in increasing importance of π bonding: 4-methylpyridine<4-vinylpyridine<pyridine<4-carboxybutyl ester<4-cianopyridine. These same authors have also noted that even if the same donor molecule and metal center are present the change of the porphyrin ligand itself can diminish the importance of π bonding. So the seemingly ambiguous findings can be simply owed to the predictable outcome of those three factors that are affecting the existence and the importance of π-bonding and can be summarized as (1) the electron configuration of the central metal; (2) the electron density of the hetero atom of the donor molecule; and (3) the nature of the porphyrin ligand itself.

In this work two different porphyrin ligands were studied with the same metal center. The equilibrium constants of Zn(TPP) and Zn(OEP) were measured as a function of temperature with five pyridine derivatives in order to investigate the major effects of five coordinate complex formation of Zn(P). The same measurements were carried out with two other bases (aceto- and benzonitrile) to get more information about the importance of π-stacking of the N-base with the zinc–porphyrin. Unlike the porphyrin complexes of open-shell paramagnetic metal ions such as Cu(II), Mn(II), Fe(II), Fe(III) or Cr(III) [16], [32], [33], [34] the complexes of closed-shell diamagnetic metal ions like Zn(II), Cd(II) or Mg(II) [35], [36], [37] are strongly fluorescent. We have taken advantage of this photophysical property of zinc(II)–porphyrins and determined the equilibrium constants of the five coordinate species not only from series of absorption but also from series of emission spectra, which enabled us to get more insight into the equilibration of the excited state porphyrin with the pyridine derivatives.

Section snippets

Materials

Synthetic Zn(TPP) and H₂(OEP) with low chlorin content, spectral grade toluene, 98% 2-picoline, 99% benzonitrile, 99% pyridine, 3- and 4-picoline and HPLC grade acetonitrile, were all purchased from Aldrich. Analytic grade ethanol and dichloromethane were obtained from Reanal while 2,6-lutidine of the same grade was obtained from Fluka. The zinc(II) derivative of OEP was prepared as it was described earlier [38], [39]. Both zinc(II)–porphyrins were further purified by dry-column chromatography

Singlet absorption and emission spectra

Metalloporphyrins are known to show well resolved absorption bands at two regions of the visible spectrum; firstly the most intense B(0,0) or Soret band is around 400 nm with ε∼10⁵ M⁻¹ cm⁻¹ while the one order of magnitude smaller Q-bands are between 500 and 650 nm, respectively. In addition, there are N, L and M bands to the blue of the B-band [33], [49]. All these absorption bands are assigned to porphyrin (π→π*) transitions. Singlet absorption and emission spectra of Zn(TPP) and Zn(OEP) in

Conclusions

The absorption spectra of the ground and excited state of Zn(TPP) and Zn(OEP) as well as the luminescence properties of these porphyrins were investigated at various temperatures and were compared. Based on the results of MRA and are in perfect agreement with literature data [10], [14], [28], [30], [56], [57]. 1:1 adduct formation was found in each case. Although the values of the equilibrium constants of Zn(TPP) and Zn(OEP) are in the same order of magnitude upon considering the same donor

Supplementary material

Stability constants for Zn(TPP) and Zn(OEP) with the seven bases at 20, 30, 40, 50 and 60°C (2 pages). Summary of data, taken from the literature, for the five-coordinate complex formation of Zn(TPP) with pyridine derivatives (1 page). Spectra of photometric titration, both from absorption and emission measurements, of Zn(OEP) (1 page). Ordering information is given on any current masthead page.

Acknowledgements

This research was supported by the Hungarian National Scientific Research Foundation through grant OTKA-023760.

References (63)

F D'Souza et al.
Inorg. Chim. Acta
(1990)
G Szintay et al.
J. Photochem. Photobiol. A
(1999)
E.A Malinka et al.
J. Photobiol. A: Chem.
(1995)
J.R Darwent et al.
Coord. Chem. Rev.
(1982)
N.C Maiti et al.
J. Photochem. Photobiol. A
(1996)
A.D Adler et al.
J. Inorg. Nucl. Chem.
(1970)
E.J Shin et al.
J. Photochem. Photobiol. A
(1997)
J Zimmermann et al.
J. Photochem. Photobiol. B
(1997)
A Horváth et al.
Inorg. Chim. Acta
(1991)
S Tobita et al.
Chem. Phys. Lett.
(1983)

L Chinsky et al.

J. Phys. Chem.

(1991)

R.F Pasternack et al.

Biochemistry

(1983)

C.B Storm et al.

J. Am. Chem. Soc.

(1966)

M.K Eggleston et al.

J. Phys. Chem. A

(1998)

T Uno et al.

Inorg. Chem.

(1997)

K.T Yue et al.

Inorg. Chem.

(1991)

M Lin et al.

Inorg. Chem.

(1993)

F D'Souza et al.

Inorg. Chem.

(1996)

F.A Walker

J. Am. Chem. Soc.

(1980)

G.C Vogel et al.

Inorg. Chem.

(1977)

F.A Walker et al.

J. Am. Chem. Soc.

(1980)

M.A Bobrik et al.

Inorg. Chem.

(1980)

G.C Vogel et al.

Inorg. Chem.

(1973)

F.A Walker et al.

J. Am. Chem. Soc.

(1975)

K Takahashi et al.

J. Phys. Chem. B

(1993)

L Jaquinod et al.

Chem. Commun.

(1998)

R.W Wagner et al.

J. Am. Chem. Soc.

(1994)

R.W Wagner et al.

J. Am. Chem. Soc.

(1996)

J Hsiao et al.

J. Am. Chem. Soc.

(1996)

J Seth et al.

J. Am. Chem. Soc.

(1996)

F. Thomas, Therapeutics, Scrip Magazine, May (1997)...

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Temperature dependence study of five-coordinate complex formation of zinc(II) octaethyl and tetraphenylporphyrin

Abstract

Introduction

Section snippets

Materials

Singlet absorption and emission spectra

Conclusions

Supplementary material

Acknowledgements

Inorg. Chim. Acta

J. Photochem. Photobiol. A

J. Photobiol. A: Chem.

Coord. Chem. Rev.

J. Photochem. Photobiol. A

J. Inorg. Nucl. Chem.

J. Photochem. Photobiol. A

J. Photochem. Photobiol. B

Inorg. Chim. Acta

Chem. Phys. Lett.

J. Phys. Chem.

Biochemistry

J. Am. Chem. Soc.

J. Phys. Chem. A

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

J. Am. Chem. Soc.

Inorg. Chem.

J. Am. Chem. Soc.

Inorg. Chem.

Inorg. Chem.

J. Am. Chem. Soc.

J. Phys. Chem. B

Chem. Commun.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.